During the past years, the interest in radio access technologies for providing services for voice, video and data has increased. There are various telecom technologies used in cellular communications. The most widespread radio access technology for mobile communication is digital cellular. Increased interest is shown in 3G (third generation) systems. 3G systems and, then, even higher bandwidth radio communications introduced by Universal Terrestrial Radio Access (UTRA) standards made applications like surfing the web more easily accessible to millions of users.
Even as new network designs are rolled out by network manufacturers, future systems which provide greater data throughputs to end user devices are under discussion and development. For example, the so-called 3GPP Long Term Evolution (LTE) standardization project is intended to provide a technical basis for radio communications in the decades to come.
To ensure competitiveness of Universal Mobile Telecommunications Service (UMTS) networks for the future, new concepts for UMTS Long Term Evolution (LTE) have been investigated. The objectives are a high-data-rate, low-latency and packet optimized radio access technology. Therefore, E-UTRA (Evolved UMTS Terrestrial Radio Access) and E-UTRAN (Evolved UMTS Terrestrial Radio Access Network) have been launched to address these objectives.
In E-UTRAN, Orthogonal Frequency Division Multiple Access (OFDMA) technology is used in the downlink. OFDM is a modulation scheme in which the data to be transmitted is split into several sub-streams, where each sub-stream is modulated on a separate sub-carrier. Hence, in OFDMA based systems, the available bandwidth is sub-divided into several resource blocks or units prior to being transmitted. A resource block may be defined in both time and frequency. According to the current assumptions in UTRAN, a resource block size is 180 KHz and 0.5 ms in frequency and time domains, respectively. The overall uplink and downlink transmission bandwidth can be as large as 20 MHz.
An overall architecture for an E-UTRAN system is shown in FIG. 1. The E-UTRAN includes one or more E-UTRAN NodeBs (eNBs) 10, providing the E-UTRAN user plane and control plane protocol terminations towards the user terminal 12. The eNBs 10 are interconnected with each other by the X2 interface 14. The eNBs 10 are also connected by the S1 interface 16 to the MME (Mobility Management Entity) 18 by the S1-MME and to the Serving Gateway (S-GW) 18 by the S1-U.
In the following, various technological aspects and features related to E-UTRAN as well as UTRAN systems are described. Regarding downlink neighboring cell measurements, in E-UTRAN, the user terminal performs a number of neighbor cell measurements. These measurements may be used for handovers, cell reselection in idle mode, etc. Examples of such measurements are Reference Symbol Received Power (RSRP) and E-UTRAN carrier Received Signal Strength Indication (RSSI). These measurements (e.g., RSRP) are generally performed on known reference signals. Carrier RSSI is however measured over all the symbols sent over the entire carrier frequency. In UTRAN, similar measurements are used for idle and for connected mode mobility. Examples of UTRAN measurements are (Common Pilot Channel) CPICH RSCP, CPICH Ec/NO and UTRA carrier RSSI, as disclosed for example in 3GPP TS 25.215, “Physical layer measurements (FDD),” the entire content of which is enclosed herewith by reference.
However, a conventional telecommunication network can support more than one carrier frequency. In that case, the user terminal is required to perform the above discussed measurements on both frequencies: intra-frequency and inter-frequency measurements. Intra-frequency neighboring cell measurements and inter-frequency neighboring cell measurements may be defined as follows. Neighboring cell measurements performed by the user terminal are intra-frequency measurements when the current and target cells operate on the same carrier frequency. Neighboring cell measurements performed by the user terminal are inter-frequency measurements when the neighboring cell operates on a different carrier frequency, compared to the current cell.
The structure of the measurement reporting to be performed by the user terminal is discussed next. The serving cell (the cell that currently serves the user terminal) generally configures the user terminal to perform neighboring cell measurements for mobility and to report the measurement reports to a base station of the serving cell. The term “base station” is used in the following as a generic term. As it is known, in the Wideband Code Division Multiple Access (WCDMA) architecture, a NodeB may correspond to the base station. In other words, a base station is a possible implementation of the NodeB. However, the NodeB is broader than the conventional base station. The NodeB refers in general to a logical node. A NodeB in WCDMA is handling transmission and reception in one or several cells. For the LTE architecture, there is a single node, the eNodeB. Although conventionally the term “base station” is narrower than the NodeB of the WCDMA architecture or the eNodeB of the LTE architecture, the term “base station” is used in the following exemplary embodiments as defining the NodeB, eNodeB or other nodes specific for other architectures. Thus, the term “base station” defined and used in the present disclosure is not limited to the conventional base station unit of a network.
There are two approaches, as disclosed in 3GPP TS 25.331, “RRC protocol specification,” the entire content of this document being incorporated by reference here, for performing the measurements. A first approach is based on a blind identification and measurement on cells performed by the user terminal and a second approach is based on received list of neighboring cells. In the first approach, the serving cell instructs the user terminal to identify and measure, and if necessary, to also report P best neighboring cells, i.e., cells from which a signal having certain parameters is detected by the user terminal. No list of the neighboring cells is provided by the serving cell to the user terminal according to this approach. The user terminal identifies and measures the best neighboring cells and reports the events and/or measurements to the serving cell.
In the second approach, the serving cell provides an explicit list of neighboring cells (R) to the user terminal and the user terminal is instructed to measure only those R cells. The list may include instructions to report only the best P neighboring cells out of the existing R cells. The cell list provided by the serving cell may include only the identity of the neighboring cells.
In WCDMA systems, both these approaches are used. However, the second approach is more suitable to WCDMA systems due to better performance requirements. In E-UTRAN systems, the first approach (i.e., without neighboring cell list) is considered to better fit the requirements of this system. However, the user terminal should apply a cell individual offset to each cell for which a measurement is performed or an event is determined as will be discussed later. The cell individual offset is specific to each cell and it is the value of this offset that the user terminal needs to apply when reporting to the serving cell an event or when taking mobility related decisions such as cell reselection, handover etc. Next, it is discussed the reporting of events and measurements by the user terminal.
In RRC_CONNECTED (Radio Resource Control) mode, the user terminal reports the measurements collected from the neighboring cell to the serving cell (i) periodically, (ii) event triggered, and/or (iii) event triggered in a periodical fashion. According to the last mechanism, after the occurrence of an event occurs (e.g., neighbor cell quality becomes x dB greater than that of the serving cell), the user terminal starts periodical reporting of the measurement(s). In addition, in the RRC_CONNECTED mode the user terminal also reports a number of events based on the performed measurements. For instance, an event is generated in the user terminal when a particular measurement quantity in a neighboring cell exceeds or falls below a threshold level. Another example of an event is when a certain measured quantity in a neighboring cell exceeds or falls below the same quantity in another neighboring cell or the serving cell. One skilled in the art would appreciate other events in UTRAN or E-UTRAN.
To perform the required measurements or determine the specific events, the user terminal, according to a first approach, needs to read the system information of each neighboring cell to be measured via the broadcast channel (BCH) to acquire the neighboring cell individual offset. Thus, the user terminal has to read the system information of all the neighboring cells for which measurement reports or associated events are to be sent to the serving cell. The drawback of this approach is the delay in reading the system information since the system information is sent with some periodicity (e.g., 40 or 80 ms or even longer) due to the structure of the broadcast channel. Another drawback is that the user terminal's processing time will increase. Furthermore, the user terminal is required to read system information even if the offset of the neighboring cell is 0 dB (i.e., there is no offset).
According to a second approach, the serving cell instructs the user terminal whether it should apply neighboring cell offsets or not when reporting the measurements and/or events. This instruction is the same for all neighboring cells. However, a drawback of this approach is in the case that the serving cell instructs the user terminal not to apply the offset. Then, the user terminal will not apply the offset even if there are cells requiring the offset. This approach may lead to inappropriate event triggering in cells with offsets and also may lead to capacity loss, as suggested in 3GPP R4-070914, “Use of cell specific offsets and reading neighbor BCH,” the entire content of which is incorporated here by reference.
A third approach is to not require the user terminal to read the system information to receive the cell offsets. Thus, in this instance, the user terminal reports measurements and events to the serving cell without any offsets. In order to achieve 0 dB offset (i.e., no offset) all cells should have the same coverage (e.g., a common channel power setting) and there should be no loss due to uplink and downlink imbalance. However, this objective is difficult to be achieved because all cells in all the coverage scenarios would not have the same coverage and/or uplink and downlink imbalances. Thus, in a practical case, where there will be non-zero offsets at least in some cells within the coverage area, the user terminal may trigger undesired events (early or delayed depending whether the offset is negative or positive), which lead to the loss in capacity.